How to know first instruction in GAS assembler?

[xiangzhc]

What I want to achieve is to calculate the length of the code segment.
In Masm Compiler, we can get it just through

Code: Select all

($-$$)

But how to get it through GAS assembler?
Here's my code, but when compiling, it returns
" Error: bad or irreducible absolute expression"

Code: Select all

.text
.global _start
#.code16

_start:
  jmp over
os_size:
  .word 0
  .word 0

over:
  movw $100, %ax

forever:
  jmp forever

foo:

.rept . - _start
  .byte 0
.endr

[Owen]

GAS isn't designed to generate binary files. You must use a linker script in order to guarantee the correct padding

[xiangzhc]

GAS isn't designed to generate binary files. You must use a linker script in order to guarantee the correct padding

Thanks for your reply. Could you please add more code here? So that I can search for that.

By the way, when linking, we can specify segment location like:

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ld -Ttext=0x0 -o bootblock bootblock.c

The abovementioned code script is taken from our assignment. What confused me is the fact that bootblock/bootloader would be loaded into 0x07c00 by BIOS, but why we specify code segment to 0x0?

Does that imply the cs register is assigned to 0x07c0 automatically?

[Combuster]

...okay...

Linking source files, asking about homework, you just broke two forum rules like really hard.


Go back to asking your teacher. You paid him after all.